1. Field of the Invention
This invention relates to a control apparatus for a DC motor, or more particularly to a speed control apparatus suitable for a DC motor for positioning control which is required to maintain a predetermined speed level during the braking and deceleration of the motor.
2. Description of the Prior Art
In the speed control of a DC motor, it is sometimes required that the motor be run at a predetermined low speed after braking and deceleration from a higher speed.
This low-speed control is necessary to stop exactly at a predetermined position a movable member of an industrial sewing machine, machine tool, elevator or the like.
As an example, explanation will be made of typical operating processes of a motor for use with the control of an industrial sewing machine with reference to the operating characteristics curve shown in FIG. 1.
In FIG. 1, the ordinate represents the number of revolutions N of the sewing machine shaft driven by the motor and the abscissa time. When the sewing machine is operating at a high speed N.sub.H after starting, the motor is released from the power supply upon the issuance of a deceleration command at point P.sub.1, whereupon the braking device is energized thereby to start deceleration.
When a point P.sub.2 representing a predetermined low speed N.sub.L is reached after t.sub.1 seconds following the issuance of the deceleration command P.sub.1, the braking action stops and the predetermined low speed N.sub.L is maintained for t.sub.2 seconds for the purpose of positioning. When the sewing machine needle reaches a predetermined position, for example, a low position P.sub.3, the position detector is energized thereby to acutate an electromagnetic brake.
Because of a certain time delay, however, this electromagnetic brake is energized actually after t.sub.3 seconds and the sewing machine needle stops at the low position after t.sub.4 seconds. In the operation of the motor having such a control mode, it is indispensable to detect that the predetermined low speed N.sub.L has been reached.
In the controlling operation of this type of motor, it will be apparent from the diagram of FIG. 1 that the motor passes through the predetermined low speed level N.sub.L at least twice during one operation mode.
For this reason, it is impossible to detect the point P.sub.2 of FIG. 1 even if the rotational speed of the motor or the sewing machine is detected by a speed generator connected to the motor. In other words, it is necessary not only to detect the speed of the motor but also to decide whether the motor is accelerating or decelerating. This makes it necessary to provide a discriminator for the above-mentioned decision as well as the speed detector, thus making the sewing machine cost very high.
For the purpose of effecting the above-described control, U.S. Pat. No. 3,573,581 discloses a device for deciding whether the motor is accelerating or decelerating according to the presence or absence of a trigger pulse of a dynamic braking thyristor connected to the motor while at the same time detecting the motor speed by converting it into a number of pulses by the use of a disc with a slit.
However, this method of detecting the braking state of the motor in response to a trigger pulse applied to the braking thyristor has the disadvantages that the thyristor may not necessarily be turned ON by a trigger pulse and that the trigger pulse may be produced by mistake, thus posing the problem of lack of accuracy and reliability.
Further, when the revolutions of the motor are decreased to the predetermined low level N.sub.L, the braking thyristor in parallel with the motor must be turned off to thereby terminate the dynamic braking and also the motor is required to be connected to the power supply again to run it at the predetermined low speed N.sub.L, say, 400 r.p.m.
In order to terminate the dynamic braking working during the deceleration, it is necessary to provide a forced commutation device for the thyristor. Also, when the power supply circuit is closed again after ascertaining that the predetermined low motor speed has been reached, the thyristor must be in a turn-off state. Otherwise, there is the danger of the power supply being short-circuited.